It is well known that spinal and epidural nerve blocks occur in humans and other animals due to a variety of causes; and such blocks can be temporary or permanent depending at least in part upon the cause.
It is likewise well known that regional anesthesia in the form of spinal and epidural nerve blocks is widely employed clinically for various surgical and non-surgical procedures to produce a temporary interruption of variable degree to various somatic sensations (which include various forms of sensations including pain sensations) and other autonomic and motor functions. Depending on the level and depth of anesthesia, spinal and eipdural nerve blocks also affect the sympathetic nervous system and can also affect the motor nervous system.
Monitoring of such nerve blocks to determine the level and/or depth of the same is obviously important, and particularly so when the block is temporarily induced.
The most common method of testing pain sensation now utilized is to strike the skin with a safety pin applying variable force. Alcohol or ether cotton swabs are also commonly used to provide information regarding the dermatominal level at which cold sensation is abolished, and this, in turn, provides information regarding the approximate level of sensory and sympathetic interruption. However, these common and simple methods are inconsistent and do not provide a monitoring system that is accurate, dependable and continuous.
The use of continuous epidural techniques is, however, desirable in situations such as long surgical procedures. This allows the injection of further doses of local anesthetic agents via an epidural catheter at intervals, or continuously, through the use of a continuous infusion apparatus. The latter case enables clinicians to maintain the continuity of a certain sensory dermatomal block level in patients under going surgery and/or for the purpose of post-operative pain relief. The use of continuous caudal and epidural anesthesia in obstetric patients for providing pain relief during labor is also relatively common. Moreover, in pain management spinal and epidural anesthesia are often used as therapeutic and diagnostic tools.
In clinical practice of regional anesthesia, it is important to regulate the level and depth of anesthetic. A low level of anesthesia could prove inadequate or risky for a particular purpose or in a particular situation. A high level and depth of anesthesia could prove dangerous as it could result in an interference with vital cardiovascular and respiratory function that could produce serious complications. Therefore, it is important to obtain accurate and continuous information regarding the depth and level of spinal and epidural anesthesia, this being vital for the patient's care.
In the area of clinical research of spinal and epidural anesthesia, such information provides a basis for a comparison, for example, between the effects of various local anesthetic agents and the resulting level and depth and duration of the anesthetic. The accuracy of the data obtained (time segment dermatome, sclerotome, sympathotome, and myotome) depends on the systems and methods available for use in monitoring the anesthetic level and depth. As used herein, time segment dermatome is defined as the spread of analgesia or sensation loss over time along the spinal segments (sacral, lumbar, thoracic, and cervical) when tested with a specific stimulus such as pin prick or surgical stimulation, time segment sympathotome is defined as the spread of sympathetic cutaneous interruption over time and along the dermatomal distribution of spinal segments (sacral, lumbar, thoracic and cervical) when tested with a specific method such as temperature changes in skin, PGR or ultrasonic skin blood flow, and time segment myotome is defined as the spread of motor block over time and along the different spinal segments (myotomal regions) when tested with a specific method such as manually examining the motor power to ultimately determine the degree, if any, of motor power loss, or through EMG measurements.
The following are some of the disadvantages relating to the existing methods that are in common use during spinal and epidural blocks:
1. The pin prick method is both inaccurate and inconsistent, since it is directly related to the force applied to the pin by the examiner. The applied force is often difficult to regulate in order to induce a quantitative and reproducible degree of sensation or pain. Different examiners may obtain different sensory block levels and depth levels testing the patient at approximately the same time;
2. The method could be unhygienic since the skin could be injured during the process of pricking. Moreover, some patients regard this as unpleasant and see it as a primitive way of testing the anesthetic depth and level;
3. During the blocking process, a situation could arise in which it becomes difficult to determine whether or not a sensory dermatomal block of sufficient depth is established. This, in part, could be due to poor cooperation by the patient as some get exhausted from continuous pricking and could eventually refuse to cooperate with the examiner. In addition, certain drugs such as tranquillizers or narcotics could modify and complicate the interpretation of the commonly used pin prick method. During the process of onset and regression of the anesthesia, areas of dermatomal hypothesia (reduced sensation) are usually developed. These areas are often difficult to quantify when compared to that of normal sensation and their development could complicate a decision as to whether or not surgery should be allowed to start. An unpleasant and risky situation could arise whereby surgery could begin based on what seemed to be acceptable evidence of sensory loss using the pin prick method, while in fact the anesthesia is not enough to counteract the surgical noxious stimulation particularly that arising from stimulation of deeper tissues below the skin level. Clinically, it is difficult to predict the exact time required to complete the block of superficial and deep sensation from that of the beginning of the onset of spinal and epidural block. The onset, progress and depths of anesthesia (sympathetic, sensory and motor block) is different at the various spinal segments. Since the pin prick method is limited in producing pain sensation compared to that produced by the surgeon's knife which elicits higher pain sensations, some clinicians prefer to use strong stimulation and also test for deep pain as well. An example is through the use of forceps pinching the skin strongly. This method, however, is not in popular use;
4. It is generally difficult and often impossible to monitor or test the anesthesia at the various dermatomal, sympathotomal and myotomal sites once surgery starts, as this could interfere with the sterility of the surgical procedure. In addition, with the patient in supine position (for example), the sacral dermatomal segments cannot be reached as these are situated in the back of the thigh and gluteal region;
5. During epidural anesthesia, and in order to maintain the sensory level of anesthetic, reinjection of local anesthetic agents through an epidural catheter is often required. Knowledge of the preinjection sensory level is important in order to regulate and determine the dosage of the anesthetic agent that could be injected further. The issue could be complicated by the fact that often the anesthetic level achieved following the start of surgery is not the final highest level as anesthesia could continue to extend to a higher level. This could result in a serious cardiovascular and respiratory complication known as total spinal or total epidural and which in turn requires emergency measures such as administration of oxygen, possible tracheal intubation, the administration of vasopressor drugs to elevate the blood pressure, intravenous fluid administration, etc.; and
6. The available methods of testing the sensory level and depth are time consuming and often do not provide the clinician with rapid and accurate information. In addition, there is no available method that could rapidly and accurately scan the sympathetic and/or motor block levels.
The sites of action of epidurally administered local anesthetics have been studied using an apparatus with a plurality of implanted electrodes, with the study being reported in an article entitled "Differential Neural Effects of Epidural Anesthetics" by Joseph F. Cusick, Joel B. Myklebust, and Stephen E. Abram, appearing in Anesthesiology, Volume 53, No. 4, pages 299-306 (October 1980).
A device and method for monitoring neuromuscular blocks present in a patient following the administration of a muscle relaxant drug which includes sensing of the movement of a patient's digital member in response to a stimulating pulse of electrical energy is shown and discussed in U.S. Pat. Nos. 4,157,087, 3,565,080 and 3,364,929.
A device and method for monitoring the degree to which the muscles of a surgical patient have been relaxed by the use of relaxant drugs which includes detecting the flexing or clenching of the fingers, due to application of an electrical stimulus, by an inflatable bladder in the patient's hand with the pressure changes being converted into electrical signals to control dispensing of the relaxant drug to the patient is shown and disclosed in U.S. Pat. No. 3,898,983.
A device and method for controlling the level of anesthesia in surgery through measurements of biocurrents is shown and described in U.S. Pat. No. 3,946,725.
An apparatus for testing the reaction time, following a stimulus, of a subject by measurement of skin resistance to make possible evaluation of the dosage of drugs affecting the central nervous system which may be safely given to particular subjects is shown and described in U.S. Pat. No. 3,468,302.
Apparatus and methods for applying electrical signals to a patient by use of a plurality of electrodes that are sequentially energized are shown and described in U.S. Pat. Nos. 4,166,452, 4,078,553 and 3,646,940.